What is it about?

Ctenophores or “comb jellies” are gelatinous marine invertebrate animals that diverged from all other living animals very early in animal evolution and are therefore important to understand what features of animal life on earth might be universal. For about 140 years, scientists have understood ctenophores to exhibit a unique life-history pattern: two phases of sexual reproduction, separated by a period of growth and development where reproduction ceases. However, in our paper “Ctenophores are direct developers that reproduce continuously beginning very early after hatching,” we found that Mnemiopsis leidyi ctenophores do not pause their reproduction so long as adequate nutritional conditions are maintained. We were watched individual Mnemiopsis, and carefully controlled their diet, culture density and temperature to count the reproductive output of individual animals throughout their life cycle. Our new results overturn the idea that ctenophores have a special kind of life cycle. We also show that the dietary omega-3 fatty acid DHA (docosahexaenoic acid) is required for spawning in Mnemiopsis and is likely important for fertility in virtually all animals. While Mnemiopsis is native to the eastern coast of North America, it is an ecologically and economically significant invasive pest in European waters. Perhaps the most important ramification of our results is that it opens the door for researchers to maintain breeding ctenophore colonies virtually anywhere in the world and will facilitate rapid progress into the molecular understanding of embryogenesis, body plan formation, and novel cell type evolution in one of the most phylogenetically pivotal taxon on Earth. This new understanding of the Mnemiopsis life cycle will be important to ecologists and fisheries managers trying to control invasive populations, and makes clear how difficult that job might be, since Mnemiopsis can routinely reproduce at microscopic sizes and starting only 10 days after their own hatching. This result will also propel Mnemiopsis to become an even more important research organism. Our lab, at the University of Florida's Whitney Laboratory for Marine Bioscience located in St. Augustine, uses the locally abundant ctenophore Mnemiopsis leidyi (also known as the “sea walnut”), which can be seen in the daytime by the rainbows from their prismatic locomotory structures and at night from their bioluminescence. Only a few places lucky enough to have marine research facilities, like the Whitney lab, have been able to do this research because these animals are extremely delicate and very difficult to maintain in laboratory culture, making coastline access essential to collect more animals as needed. However, this new information about their life cycle will allow laboratory culture of these animals anywhere, and will allow us to maintain genetic lineages and perform multi-generational, transgenic experiments.

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Why is it important?

The practical implications for this research will be in two areas: 1. Making work with ctenophores more accessible to researchers located in many places, not just marine labs. This is important because biologists are increasingly interested in ctenophores for understanding animal regeneration following injury, basic insights into how the nervous system works, and how things like regeneration and nervous systems evolved in the first place. Knowing this will be important for knowing what is likely universal, and therefore can potentially be used to help humans. A shorter life cycle and ease of lab culture is something researchers look for in lab organisms because it enables genetic experiments. 2. While these animals are native where we are, they are invasive elsewhere, with major ecological and economic consequences, particularly for fisheries. This paper shows that they are reproductive before they would ever be visible to the naked eye, and that things like warmer temperatures and particular types of algae their prey eat greatly increase their reproductive ability. Understanding their basic reproductive biology is important to controlling these and other invasions, particularly in a warming world.

Perspectives

The story of how this paper was written really shows how much luck can matter in science, and how the pandemic has shaped the science I am doing. First, I only noticed that we had spawning small-size animals due to several lucky factors (a broken air conditioner, unusual culture conditions we were using for a totally unrelated experiment). Since I knew it was an under-researched phenomenon, I jumped at the opportunity to take a look when it presented itself, but that’s a bit of a risk and rare opportunity to be able to jump on something that might be a distraction, so I feel lucky that my lab mentor, Mark Martindale, supported me in doing that. I’m also very lucky that I was able to bring my animals and a microscope home with me during the very early lockdown phase of the COVID pandemic so I was able to continue some of this work from home. Keeping that momentum going was really helpful – maybe I would have dropped the project in favor of computational work or something else easier to do from home otherwise. It’s also really important to know what you don’t know. I originally did all the statistics myself using standard methods I already knew, and I essentially got the same answers that are in the final paper in terms of which factors are important, but I also knew that the distribution of my data did not truly fit the assumptions of the statistics I was using. Getting a statistician on board who could develop a more rigorous approach that fit the data was an incredible opportunity for me to learn new approaches and to make the paper as good as it could be. Again, if not for COVID normalizing video calls with collaborators, bringing José Miguel Ponciano on board might have been more difficult since I am at the marine station rather than UF’s main campus where he is. None of this is to say that the pandemic has been a net good for anyone, it’s obviously not at all, but it absolutely has shaped this project.

Allison Edgar
The Whitney Laboratory for Marine Bioscience, University of Florida

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This page is a summary of: Ctenophores are direct developers that reproduce continuously beginning very early after hatching, Proceedings of the National Academy of Sciences, April 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2122052119.
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